The present invention relates to a system for providing pressurized fluid to a plurality of load circuits (fluid pressure operated devices), and more particularly, to such a system in which one of the load circuits is given priority, with all flow not being used by the priority load circuit going to the other (auxiliary) load circuit.
The present invention is equally adapted to any arrangement in which pressurized fluid is communicated to a priority load circuit, and an auxiliary load circuit, by a load sensing priority flow control valve, in response to a load pressure signal indicating the demand for fluid by the priority load circuit. However, the invention is especially advantageous in arrangements in which the priority load circuit comprises a vehicle hydrostatic power steering system, and the invention will be described in connection therewith.
Load sensing priority flow control systems have become increasingly popular for use in many applications to provide pressurized fluid to multiple load circuits from a single source, partly to make the overall system less expensive, and partly to minimize the energy consumption (i.e., the load on the vehicle engine). The earliest known load sensing priority flow control system is illustrated and described in U.S. Pat. No. 3,455,210, assigned to the assignee of the present invention, and incorporated herein by reference. A later teaching of such a system is contained in U.S. Pat. No. 4,043,419, also assigned to the assignee of the present invention, and incorporated herein by reference. In this latter patent, there is a specific teaching of the priority load circuit comprising a hydrostatic power steering device. Furthermore, there is a specific teaching of any particular load sensing priority flow control valve and hydrostatic power steering unit combination being used with any one of the following:
(1) a fixed displacement pump and an open center auxiliary device; or, PA0 (2) a pressure compensated pump and closed center auxiliary device; or, PA0 (3) a load sensing pump and a load sensing auxiliary device.
The ability of load sensing priority flow control systems to provide improved performance with substantially reduced engine horsepower consumption is so substantial that, for a number of years, the manufacturers of most agricultural and construction type vehicles have been converting from the traditional open center and closed center systems to load sensing systems. However, the conversion to load sensing systems has not been made universally, primarily because of the cost associated with the load sensing priority valve itself, and the associated plumbing between the priority valve and the priority load circuit, and between the priority valve and the auxiliary load circuit. It has long been assumed, however, that the extra costs required to provide a load sensing system are basically unavoidable.